1. Field of the Invention
This invention relates to radar generally and, more specifically, to receiver systems for pulse radar providing beam angle discrimination.
2. Description of the Prior Art
Many operational tracking radars work on the principle of "simultaneous lobing" or "monopulse". By assuming that the radar antenna is a parabolic dish, and restricting attention to elevation angle measurements, the principle can be outlined as follows. The target is illuminated by a single transmit beam along the axis of the parabola. On reception, the signal is picked up by two antenna elements (horns, for example) on the focal plane displaced either side of the axis. A balance between echo signals received on these two elements indicate that the antenna, as a whole, is pointing directly at the target, since the two horns are equally (symmetrically) illuminated. This is brought about by forming the difference .DELTA. between the two signals and also the sum .SIGMA. of the signals, and then providing an output to .DELTA./.SIGMA. (difference/sum ratio), indication of the pointing error which is (ideally) independent of absolute amplitude. From this error signal, a control loop can be operated to mechanically adjust the pointing angle of the dish so that the difference signal reduces to zero, the arrangement then being called a tracker.
With targets well above the horizontal, it is possible to achieve very high accuracies with this approach, however, conventional monopulse trackers experience problems when attempting to follow a low flying target accurately and without instability. In such a low angle situation, the image of the target return caused by ground reflection interferes severely with the direct return and results in a rapid degradation of performance below one beamwidth elevation (typically one degree) leading to loss of reliable data below about 0.5 beamwidth elevation. In the latter condition, tracking instability occurs because the pointing angle can alternate between that corresponding approximately to the image and that of the target.
Another factor is that the lowest angles often occur at the longest target ranges when signal above noise is at a premium. Known techniques for improving angular resolution often involved a large "expenditure" of signal-to-noise ratio. Further, the elevation lobing of the transmitting pattern (caused again by ground reflection) can significantly reduce the power on target, sometimes resulting in a near-nulling (phase cancellation) of the illumination.
In consideration of the aforementioned state of this art, it may be said to be an object of the invention to reduce the overall power budget degradation at low angles while still enabling the angular resolution and accuracy to be maintained at a high level.
According to the invention, there is provided a receiving system for a pulse radar system comprising range gate controlled means for simultaneously sampling the return signal distribution across a predetermined portion of the receiving aperture, apparatus for combining these samples to produce a coherent signal waveform corresponding to a time scan of the return signal distribution across that predetermined portion of aperture and having a signal spectrum which is representative of the aperture distribution, and also means for filtering the received waveform so as to attenuate the edges of the signal spectrum.
The invention will be better understood from the following detailed description made in conjunction with the accompanying drawings.